The armamentarium of pharmacotherapeutics includes agonists (receptor activators), and antagonists (inhibitors of receptor activation). However, many drugs fall in between these two categories and are classified as partial agonists. In therapeutic applications, a partial agonist can act as a sort of activity buffer for naturally occurring strong agonists, providing receptor stimulation if the natural activator is low or absent, and diminishing the effects of a natural strong activator if the strong activator's concentrations are high.
Partial agonists may also be used to buffer the effects of drugs of abuse; an area of current interest for the development of partial agonist therapies is for the treatment of nicotine addiction and dependence. A new drug in this area is the cytisine-related compound, varenicline (®Chantix). Varenicline is a weak partial agonist for nicotinic acetylcholine receptor (nAChR) subtypes in the brain that contain α4 and β2 subunits, which have been shown in animal studies to be essential for nicotine to stimulate the brain's neurochemical reward systems. However, varenicline will also further decrease the function of α7 receptors in neuropsychiatric patients. Reports of suicide and worsened depressions in patients taking varenicline has led the FDA to issue a black box warning on the drug. Thus, using varenicline by itself is not a satisfactory solution for many individuals suffering from nicotine dependence. Consequently, other solutions are desirable.